Thermostable biocatalyst combination for nucleoside synthesis

ABSTRACT

A recombinant expression vector comprising: a) the sequence encoding a purine nucleoside phosphorylase (PNPase, E. C. 2.4.2.1), b) the sequence encoding a uridine phosphorylase (UPase, E. C. 2.4.2.3), c) or both; each of the sequences operably linked to one or more control sequences that direct the production of said phosphorylases in a suitable expression host; said sequences originating from the Archaea Thermoprotei class, characterized in that the PNPase is from  Sulfolobus solfataricus  (SEQ ID NO. 7) and the UPase is from  Aeropyrum pernix  (SEQ ID NO. 8). In addition, the present invention relates to A transglycosylation method between a sugar-donating nucleoside and an acceptor base in the presence of phosphate ions, characterised in that said method comprises the use of a uridine phosphorylase (UPase) of  Aeropyrum pernix  (NC_000854.2), a purine nucleoside phosphorylase (PNpase) of  Sulfolobus solfataricus  (NC_002754.1), or a combination thereof.

FIELD OF THE INVENTION

The invention belongs to the field of biotechnology.

BACKGROUND OF THE INVENTION

(Deoxy)nucleosides are glycosylamines consisting of a base like a purineor a pyrimidine bound to a ribose or deoxyribose sugar, the latter beingcyclic pentoses. Examples of these include cytidine, uridine, adenosine,guanosine, thymidine, and inosine. Nucleoside analogues are extensivelyused as antiviral and anticancer agents because of their ability to actas reverse transcriptase inhibitors or chain terminators in RNA or DNAsynthesis [1].

Chemical synthesis of nucleoside analogues has been achievedstereoselectively but using expensive or polluting reagents [2] andinvolving multistage processes that can be time consuming. Biocatalyticprocedures offer a good alternative to the chemical synthesis ofnucleosides because biocatalyzed reactions are regio- andstereoselective and allow the decrease of by-products content. Ofparticular interest within the biocatalytic procedures is the enzymatictransglycosylation between a sugar-donating nucleoside and an acceptorbase by means of enzymes that catalyse the general reversible reactions[3] as depicted in FIGS. 1 and 2.

Nucleoside phosphorylases are transferases widely distributed inmammalian cells and bacteria and play a central role in the nucleosidemetabolism salvage pathway. They have a dual functionality. On the onehand, they catalyse the reversible cleavage of the glycosidic bond ofribo- or deoxyribo nucleosides in the presence of inorganic phosphate inorder to generate the base and ribose- or deoxyribose-1-phosphate. Theseenzymatic reactions employing the purine nucleoside phosphorylases andthe pyrimidine nucleoside phosphorylases are shown in FIG. 1. On theother hand, these enzymes catalyse phosphate-dependent pentose transferbetween purine or pyrimidine bases and nucleosides, i.e.transglycosylation reactions, to produce nucleosides with differingbases. FIG. 2 shows an example of a one-pot synthesis using nucleosidephosphorylases.

When the pyrimidine and purine nucleoside phosphorylases are used incombination, it is possible to transfer the sugar from a donorpyrimidine nucleoside to a purine or pyrimidine acceptor base as well asfrom a donor purine nucleoside to a pyrimidine or purine acceptor base,depending on the starting materials used [4]. As a consequence,nucleoside phosphorylases from different sources, mainly bacterial, havebeen exploited as tools for the enzymatic synthesis of nucleosideanalogues.

In nature these enzymes have been described in various microbialstrains, particularly in thermophilic bacteria (i.e. bacteria thrivingat temperatures between 45° C. and 80° C.), which have been used assources of nucleoside phosphorylases in numerous works for obtainingmodified nucleosides by enzymatic transglycosylation. However, althoughin these studies the target products yields were sufficiently high, theamount or ratio of the enzymatic activities necessary fortransglycosylation was non-optimal [5]. They required either aconsiderable extension in the reaction time (up to several days) or anincrease in the used bacterial biomass to reach the necessarytransformation depth.

Besides, when developing a transglycosylation process another problemarises: the difficult solubilization of large amounts of substrates andproducts, many of them poorly soluble in aqueous medium at roomtemperature. Although this problem could be solved using highertemperatures, it requires enzymes sufficiently stable in these harderreaction conditions.

The Archaea are a group of single-celled microorganisms that are one ofthe three domains of life; the others being Bacteria and Eukarya. Theywere formerly called Archaebacteria under the taxon Bacteria, but noware considered separate and distinct. The archaeal domain is currentlydivided into two major phyla, the Euryarchaeota and Crenarchaeota. TheEuryarchaeota includes a mixture of methanogens, extreme halophiles,thermoacidophiles, and a few hyperthermophilcs. By contrast, theCrenarchaeota includes only hyperthermophiles. Hyperthermophiles arethose organisms that thrive in extremely hot environments, from 60° C.upwards, optimally above 80° C.

Cacciapuoti et al. [6-8] describe two purine nucleoside phosphorylases(PNPases) from hyperthermophilic Archaea, in particular it discloses theenzymes 5′-deoxy-5′-methylthioadenosine phosphorylase II (SsMTAPII, EC2.4.2.28) from Sulfolobus solfataricus, and purine nucleosidephosphorylase (PfPNP) from Pyrococcus furiosus. The Pyrococcus furiosusenzyme was firstly annotated as MTAPII but renamed to PNP as it isunable to cleave methylthioadenosine. Sulfolobus solfataricus belongs tothe Crenarchaeota, while Pyrococcus furiosus belongs to theEuryarchaeota. The EC code above is the conventional enzyme nomenclatureprovided by the International Union of Biochemistry and MolecularBiology that classifies enzymes by the reactions they catalyse.

Most enzymes characterized from hyperthermophiles are optimally activeat temperatures close to the host organism's optimal growth temperature.When cloned and expressed in mesophilic hosts like Escherichia coli,hyperthermophilic enzymes usually retain their thermal properties.Sometimes the enzymes are optimally active at temperatures far above thehost organism's optimum growth temperature [9]. Other times enzymes havebeen described to be optimally active at 10° C. to 20° C. below theorganism's optimum growth temperature [10-11]. However, the Sulfolobussolfataricus 5′-methylthioadenosine phosphorylase (a hexameric enzymecontaining six intersubunit disulfide bridges), when expressed in amesophilic host, forms incorrect disulfide bridges and is less stableand less thermophilic than the native enzyme [12].

The Thermoprotei are a hyperthermophilic class of the Crenarchaeota.From the genomes sequenced and available for the Archaea Thermoproteiclass, only three sequences for purine-nucleoside phosphorylase (EC2.4.2.1) and only three sequences for uridine phosphorylase (EC2.4.2.3), were found. These six proteins have been entered,respectively, in UniProtKB/TrEMBL with the accession numbers: A1RW90(A1RW90_THEPD), for the hypothetical protein from Thermofilum pendens(strain Hrk 5); Q97Y30 (Q97Y30_SULSO), for the hypothetical protein fromSulfolobus solfataricus; A3DME1 (A3DME1_STAMF), for the hypotheticalprotein from Staphylothermus marinus (strain ATCC 43588/DSM 3639/F1);Q9YA34 (Q9YA34_AERPE), for the hypothetical protein from Aeropyrumpernix; A2BJ06 (A2BJ06_HYPBU) for the hypothetical protein fromHyperthermus butylicus (strain DSM 5456/JCM 9403); and D9PZN7(D9PZN7_ACIS3) for the hypothetical protein from Acidilobussaccharovorans (strain DSM 16705/VKM B-2471/345-15). All these sequenceswere under the annotation status of unreviewed, which means that theirpresence in the Archaea has only been verified by computer.

Even though many genes can be successfully expressed in Escherichia coliat high yields, several proteins from hyperthermophiles are poorly ornot at all expressed, partially due to the usage of rare codons. Indeed,and to the best of our knowledge, no party was yet successful inexpressing any of the mentioned genes above.

In view of the prejudices above, in view of the technical difficulties,the inventors unexpectedly were able to prepare viable recombinantvectors and importantly, obtain recombinant phosphorylases that wereoptimally active at temperatures higher than 60° C. The thermostable andchemically stable catalysts of the present invention are a purinenucleoside phosphorylase (PNPase, E.C. 2.4.2.1), and a uridinephosphorylase (UPase, E.C. 2.4.2.3), originating from the ArchaeaThermoprotei class, wherein the PNPase is from Sulfolobus solfataricus(SEQ ID NO. 7) and the UPase is from Aeropyrum pernix (SEQ ID NO. 8).

In particular, it has been surprisingly found that the recombinantnucleoside phosphorylases derived from the hyperthermophilicThermoprotei have unique structure-function properties like enhancedthermostability, high catalytic efficiency, and optimal enzymaticactivities at temperatures near or above 100° C. These recombinantenzymes can advantageously be used for transglycosylation reactions, inthe form of cell lysate and in the form of crude or purified extracts,for industrial production of natural and modified nucleoside analogues.They are in particular versatile since they can catalyzetransglycosylations in aqueous media, in organic solvents, attemperatures between 60° C. and 120° C., or in a combination of theseparameters, allowing the preparation of many and diverse types ofnucleosides at acceptable production yields, reaction times, andemploying economical amounts of the enzymes. Importantly, thebiocatalysts described in the present invention can be used forbioconversion reactions that require the presence of organic solvents,temperatures above 60° C., or both, in order to solubilize thesubstrates or the reaction products. These phosphorylases are ideal inreactions with water-insoluble substrates. Another advantage of thesephosphorylases resides in their organic solvent tolerance, and in thatthey can be reused for several reaction cycles.

More advantageously, the invention offers a combination of Thermoproteinucleoside phosphorylases that is useful for one-pot synthesis ofnucleosides. The enzymes can be used to produce natural or analognucleosides in a one-step (one-pot) or two-step synthetic methods. Inthe one-step synthesis, a pyrimidine nucleoside phosphorylase and apurine nucleoside phosphorylase are used in the same batch in order tochange the base linked to the sugar by another one of choice. In the twostep, a pyrimidine nucleoside phosphorylase is used for the liberationof the sugar of a pyrimidine nucleoside, and then, the 1-phosphate-sugaris isolated and later on, in another vessel, a purine base is linked tothe sugar using a purine nucleoside phosphorylase.

SUMMARY OF THE INVENTION

The present invention relates to a recombinant expression vectorcomprising: a) the sequence encoding a purine nucleoside phosphorylase(PNPase, E.C. 2.4.2.1), b) the sequence encoding a uridine phosphorylase(UPase, E.C. 2.4.2.3), c) or both; each of the sequences operably linkedto one or more control sequences that direct the production of saidphosphorylases in a suitable expression host; said sequences originatingfrom the Archaea Thermoprotei class, characterized in that the PNPase isfrom Sulfolobus solfataricus (SEQ ID NO. 7) and the UPase is fromAeropyrum pernix (SEQ ID NO. 8).

In addition, the present invention relates to A transglycosylationmethod between a sugar-donating nucleoside and an acceptor base in thepresence of phosphate ions, characterised in that said method comprisesthe use of a uridine phosphorylase (UPase) of Aeropyrum pernix (NationalCenter for Biotechnology Information Reference Sequence:NC_(—)000854.2), a purine nucleoside phosphorylase (PNPase) ofSulfolobus solfataricus (NCBI RefSeq: NC_(—)002754.1), or a combinationthereof.

DESCRIPTION OF THE FIGURES

FIG. 1 shows an example of two enzymatic reactions catalyzed bynucleoside phosphorylases. The first reaction on top is a phosphorolysisthat takes place through an S_(N)1-like mechanism via an oxonium-likeintermediate to give α-ribose-1-phosphate. The second reaction occursthrough an S_(N)2 mechanism where phosphate is substituted by a baseaffording the β-nucleoside [13]. In the scheme, uridine nucleosidephosphorylase catalyzes the phosphorolytic cleavage of the C—Nglycosidic bond of uridine resulting in ribose-1-phosphate and uracil.The purine nucleoside phosphorylase (adenosine nucleoside phosphorylase)catalyzes the cleavage of the glycosidic bond, in the presence ofinorganic orthophosphate (P_(i)) as a second substrate, to generate thepurine base and ribose(deoxyribose)-1-phosphate. For the naturalsubstrates, the reactions are reversible.

FIG. 2. Scheme of one-pot synthesis using nucleoside phosphorylaseenzymes.

FIG. 3 shows a genetic map of the initial expression vectorpET102/D-TOPO® before cloning. Vector length of 6315 nucleotides. T7promoter: bases 209-225; T7 promoter priming site: bases 209-228; lacoperator (lacO): bases 228-252; ribosome binding site (RBS): bases282-288; His-patch (HP) thioredoxin ORF: bases 298-627; TrxFus forwardpriming site: bases 607-624; EK recognition site: bases 643-657; TOPO®recognition site 1: bases 670-674; overhang: bases 675-678; TOPO®recognition site 2: bases 679-683; V5 epitope: bases 700-741;polyhistidine (6×His) region: bases 751-768; T7 reverse priming site:bases 822-841; T7 transcription termination region: bases 783-911; blapromoter: bases 1407-1505; Ampicillin (bla) resistance gene (ORF): bases1506-2366; pBR322 origin: bases 2511-3184; ROP ORF: bases 3552-3743(complementary strand); lacI ORF: bases 5055-6146 (complementarystrand).

FIG. 4 depicts a “Doehlert Matrix” where five temperatures were combinedwith three pH values, resulting in seven combinations of temperature andpH.

FIG. 5. Contour plot showing the interactive effect of pH andtemperature on the UPase activity. Data was statistically analyzed withthe Response Surface Methodology (RSM), using the “Minitab” software.The enzyme appears highly thermophilic; its activity increased sharplyup to the maximal assayed temperature (100° C.) and the activitydisplayed a distinct pH optimum around the neutrality (6.5-7.5),preferably 7.0.

FIG. 6. Contour plot showing the interactive effect of pH andtemperature on the PNPase activity. Data was statistically analyzed withthe Response Surface Methodology (RSM), using the “Minitab” software.The enzyme appears highly thermophilic; its activity increased sharplyup to the maximal assayed temperature (100° C.) and the activitydisplayed a distinct pH optimum around the neutrality (6.5-7.0).

FIG. 7 depicts the DNA sequence (SEQ ID NO. 7) of the coding region ofthe purine nucleoside phosphorylase (PNPase) of Sulfolobus solfataricus,a.k.a. deoD gene. GenBank accession number AE006766.

FIG. 8 depicts the DNA sequence (SEQ ID NO. 8) of the coding region ofthe pyrimidine nucleoside phosphorylase (UPase) of Aeropyrum pernix,a.k.a. udp gene. GenBank accession number NC000854.

DESCRIPTION OF THE INVENTION

The present invention relates to a recombinant expression vectorcomprising: a) the sequence encoding a purine nucleoside phosphorylase(PNPase, E.C. 2.4.2.1), b) the sequence encoding a uridine phosphorylase(UPase, E.C. 2.4.2.3), c) or both; each of the sequences operably linkedto one or more control sequences that direct the production of saidphosphorylases in a suitable expression host; said sequences originatingfrom the Archaea Thermoprotei class, characterized in that the PNPase isfrom Sulfolobus solfataricus (SEQ ID NO. 7) and the UPase is fromAeropyrum pernix (SEQ ID NO. 8).

Aeropyrum pernix and Sulfolobus solfataricus are hyperthermophilicArchaea capable of growing at high temperatures, over 90° C. Archaea areorganisms belonging to a third group of organisms distinct fromeukaryotes and prokaryotes. They are considered to descend from primevalorganisms, and are special organisms which have neither evolved noradapted to ordinary temperature environments.

UPase and PNPase in their intracellular natural environment do not allowthe synthesis of nucleoside or nucleoside analogues with high yield asdesired at industrial level. To overcome this serious limitation,inventors have used recombinant DNA technology to design an expressionvector comprising udp and deoD genes and appropriate elements toover-express nucleoside phosphorylases in selected hosts, like bacteria.The designed expression vector also facilitates the solubilization andthe purification of the different phosphorylases.

The vectors of the present invention comprise a nucleotide sequenceencoding different nucleoside phosphorylases and nucleotide sequencesthat allow said vector to be selectable and autonomously replicable inthe host cell.

The construction of the recombinant expression vector is carried outusing conventional recombinant DNA technologies, i.e. procedures to jointogether DNA segments in a cell-free system.

The term “vector” refers to a DNA molecule originating from a virus, aplasmid, or the cell of a higher organism in which another DNA fragmentof appropriate size can be integrated (cloned) without loss of thevector capacity for self-replication. Examples are plasmids, cosmids,and yeast artificial chromosomes. Vectors are often recombinantmolecules containing DNA sequences from several sources. The term“expression vector” means a vector that further comprises the necessarycontrol or regulatory sequences to allow transcription and translationof the cloned gene or genes. Circular or linearized DNA vectors areuseful for this invention.

To allow the vector of the invention to be selectable and autonomouslyreplicable in host cells, the selected vector must be compatible withthe selected host cells. In a preferred embodiment, the nucleotidesequence which allows said vector to be selectable and autonomouslyreplicable in Escherichia coli is the T7 promoter-encoding gene whichpermits the T7 RNA polymerase of the selected strain of Escherichia colito bind to the promoter. The term “selectable” means that the vectorremains stable in the descendent bacteria. The selection is achieved bystringent medium conditions according to the introduction of anappropriate selectable marker gene in the vector whose expression allowsone to identify cells that have been transformed with the vector. Theselectable marker gene is often an antibiotic-resistant gene. Preferredselectable marker genes for this invention are kanamycin, tetracycline,carbenicillin and more preferably, ampicillin.

The present invention further relates to a host cell comprising any oneof the recombinant expression vectors mentioned above, or bothrecombinant expression vectors within the same host cell.

The term “host cell” refers to a cell transformed with the recombinantexpression vector that comprises the PNPase or UPase nucleotidesequence. Another aspect of the recombinant DNA vector allows the hostcell to produce nucleoside phosphorylases, and when medium conditionsare suitable, said nucleoside phosphorylases catalyze the obtention ofnucleosides. In a particular embodiment of the invention, PNPase andUPase genes from Sulfolobus solfataricus and Aeropyrum pernix,respectively, were introduced in the DNA expression vector. FIG. 7 andFIG. 8 list nucleic acid and amino acid sequences relevant to theinvention, namely the nucleic acid sequence of Sulfolobus solfataricusdeoD (SEQ ID NO. 7) and nucleic acid sequence of Aeropyrum pernix udp(SEQ ID NO. 8), respectively.

Those skilled in the art will appropriately choose the expression systemconstituted by an initial vector and a host cell strain to maximize theproduction of nucleosides.

In one embodiment, the host cell is Escherichia coli.

In a particular embodiment, the Escherichia coli belongs to BL21bacterial strain. Suitable expression vectors for Escherichia coli BL21are for instance pET vectors, trcHis vectors and pUB vectors (all ofthem from Invitrogen), and pGEX vectors and GST vectors (from Amersham).Escherichia coli DH5 alfa bacterial strain in combination with pUCvectors and Escherichia coli F′ in combination with PSL vectors, PEZZvectors or M13 vectors (all of them from Amersham) are also useful inthis invention.

In one embodiment, the host cell is processed or is in the form of alysate.

The present invention further relates to a transglycosylation methodbetween a sugar-donating nucleoside and an acceptor base in the presenceof phosphate ions, characterised in that said method comprises the useof a uridine phosphorylase (UPase) of Aeropyrum pernix (NC_(—)000854.2),a purine nucleoside phosphorylase (PNPase) of Sulfolobus solfataricus(NC_(—)002754.1), or a combination thereof.

The term “sugar-donating nucleoside” refers to a glycosylamineconsisting of a nucleobase (often referred to as simply base) bound to aribose or deoxyribose sugar via a beta-glycosidic linkage. Examples of“sugar-donating nucleosides” include, without being limited to,cytidine, uridine, adenosine, guanosine, thymidine and inosine, as wellas those natural or modified nucleosides containing D-ribose or2′-deoxyribose; nucleosides containing the ribose group modified in the2′, 3′, and/or 5′ positions; and nucleosides in which the sugar isbeta-D-arabinose, alpha-L-xylose, 3′-deoxyribose, 3′,5′-dideoxyribose,2′,3′-dideoxyribose, 5′-deoxyribose, 2′,5′-dideoxyribose,2′-amino-2′-deoxyribose, 3′-amino-3′-deoxyribose, or2′-fluoro-2′-deoxyribose.

The term “acceptor base” refers to a nucleobase, nucleotide base,nitrogenous base, or simply base. In nature, bases are part of DNA orRNA. The primary nucleobases are cytosine, guanine, adenine (DNA andRNA), thymine (DNA) and uracil (RNA), abbreviated as C, G, A, T, and U,respectively. The term “acceptor base” in the present invention is meantto comprise also modified and analog nucleobases. In DNA, the mostcommon modified base is 5-methylcytidine (m5C). In RNA, there are manymodified bases, including pseudouridine (Ψ), dihydrouridine (D), inosine(I), ribothymidine (rT) and 7-methylguanosine (m7G). Hypoxanthine andxanthine are two of the many bases created through mutagen presence.Other examples of acceptor bases include natural or substitutedpyrimidine and purine bases; purine bases substituted at one or more ofthe 1, 2, 6 positions; pyrimidine bases substituted at one or more ofthe 3, 5 positions; and purine, 2-azapurine, 8-azapurine, 1-deazapurine(imidazopyridine), 3-deazapurine, 7-deazapurine, 2,6-diaminopurine,5-fluorouracil, 5-trifluoromethyluracil, trans-zeatin,2-chloro-6-methylaminopurine, 6-dimethylaminopurine, 6-mercaptopurine.

This transglycosylation method is useful for the preparation ofnucleosides, nucleosides analogs, and particularly active pharmaceuticalingredients (API); comprising, containing, or consisting of nucleosidemoieties, or analogs thereof. Understanding API as any substance ormixture of substances intended to be used in the manufacture of drug(medicinal) product and that, when used in the production of a drug,becomes an active ingredient of the drug product. Such substances areintended to furnish pharmacological activity or other effect in thediagnosis, cure, mitigation, treatment, or prevention of disease or toaffect the structure and function of the body. (Eudralex, Part II ofvolume 4 EU Guidelines to Good Manufacturing Practice).

The combination of uridine phosphorylase (UPase, E.C. 2.4.2.3) andpurine nucleoside phosphorylase (PNP; E.C. 2.4.2.1) efficientlytransfers a sugar moiety from a donor nucleoside to an acceptor base.

When pyrimidine nucleosides are prepared departing from other pyrimidinenucleosides and pyrimidine bases as starting materials, then the use ofthe UPase alone is sufficient, but the use of both enzymes PNPase andUPase is preferred because the PNPase can also contribute to thephosphorolysis step. Conversely, when purine nucleosides are prepareddeparting from other purine nucleosides and purine bases as startingmaterials, then the use of both PNPase and UPase is also preferred. Onthe other hand, the use of both enzymes PNPase and Upase is much moresuccessful when the reaction is from a pyrimidine to a purinenucleoside, for instance from a uridine to a 2,6 diaminopurine riboside,when compared to the use of each type of enzyme per separate.

Preferably the transglycosylation method uses a combination of the UPaseand PNPase. The crude cell lysates or the clarified crude enzymesolutions may be mixed in different proportions in order to obtain anoptimized biocatalyst for a particular transglycosylation reaction.

In one embodiment, in the transglycosylation method of the presentinvention, the UPase of Aeropyrum pernix and the PNPase of Sulfolobussolfataricus are provided by a host cell according to any one of theembodiments presented hereinbefore and hereinafter.

In one embodiment, in the transglycosylation method of the presentinvention, the UPase, the PNPase, or a combination thereof, are used inthe form of a lysate.

In one embodiment, the transglycosylation method comprises the steps of:(i) culturing the host cell in a suitable culture medium; (ii)overexpressing the UPase, PNPase, or both; (iii) optionally preparing acell lysate; (iv) adding a sugar-donating nucleoside, an acceptor base,and phosphate ions, and (v) recovering nucleosides from the reactionmixture.

In a particular embodiment the Escherichia coli transformant comprisingthe vector may be grown in a culture medium comprising tryptone, yeastextract, sodium chloride and an antibiotic selected from the groupconsisting of kanamycin, tetracycline, carbenicillin, and ampicillin,preferably at 37° C., to an optical density between 0.5-0.8 at awavelength of approximately 600 nm. The culture may be then added withisopropyl-beta-D-thiogalactopyranoside (IPTG) to a final concentrationof 100 mg/l and the inductions may be done at 37° C. between 6-12 h.Cells may be harvested by centrifugation at 4° C. and cell pellets maybe lysed by three freeze-thaw cycles. The recombinant host cells may bedisrupted by standard techniques known to those skilled in the art. Theresulting cell lysate may be directly used as biocatalyst or centrifugedto remove cell debris and to obtain a clarified crude enzyme solutions.As used herein, the term “biocatalyst” refers to any biological entitycapable of catalyzing the conversion of a substrate into a product, inthis case the nucleoside biotransformation.

In one embodiment, in the transglycosylation method of the presentinvention, the sugar-donating nucleoside is selected from natural ormodified nucleosides containing D-ribose and 2′-deoxyribose; nucleosidescontaining the ribose group modified in the 2′, 3′ and/or 5′ positions;and nucleosides in which the sugar is beta-D-arabinose, alpha-L-xylose,3′-deoxyribose, 3′,5′-dideoxyribose, 2′,3′-dideoxyribose,5′-deoxyribose, 2′,5′-dideoxyribose, 2′-amino-2′-deoxyribose,3′-amino-3′-deoxyribose, 2′-fluoro-2′-deoxyribose.

In one embodiment, in the transglycosylation method of the presentinvention, the acceptor base is selected from natural or substitutedpyrimidine and purine bases; purine bases substituted at the 1, 2, 6positions, or a combination thereof of the purine ring; pyrimidine basessubstituted at the 3, 5 positions, or a combination thereof of thepyrimidine ring; for instance purine, 2-azapurine, 8-azapurine,1-deazapurine (imidazopyridine), 3-deazapurine, 7-deazapurine,2,6-diaminopurine, 5-fluorouracil, 5-trifluoromethyluracil,trans-zeatin, 2-chloro-6-methylaminopurine, 6-dimethylaminopurine,6-mercaptopurine.

In one embodiment, in the transglycosylation method of the presentinvention, the resulting nucleoside analogue is an active pharmaceuticalingredient (API) as known in the art.

In one embodiment, in the transglycosylation method of the presentinvention, said method is carried out between 60 and 100° C.

In one embodiment, the transglycosylation method of the presentinvention is carried out in aqueous media, or in an aprotic polarco-solvent system.

In one embodiment, the aprotic polar co-solvent system is selected fromdimethylsulfoxide, tetrahydrofuran, 2-methyltetrahydrofuran,dimethylformamide, or any combination thereof.

In one embodiment, in the transglycosylation method of the presentinvention, the sugar-donating nucleoside is selected from uridine and2′-deoxyuridine, and the acceptor base is 2,6-diaminopurine.

In one embodiment, the present invention provides a one-pot enzymaticsynthesis of nucleosides using uridine or 2′-deoxyuridine as donors ofsugar moiety, because the recombinant UPase enzyme of the presentinvention is most specific for these substrates. Nevertheless, thisenzyme can be used with any donor of sugar moieties because it does notdiscriminate between uridine, 2′-deoxyuridine, and other pyrimidinenucleosides, as unfortunately occurs in many lower organisms, such as B.stearothermophilus [14].

In one embodiment, in the transglycosylation method of the presentinvention, the sugar-donating nucleoside is selected from uridine and2′-deoxyuridine, and the acceptor base is 5-fluorouracil.

In one embodiment, in the transglycosylation method of the presentinvention, the sugar-donating nucleoside is selected from uridine and2′-deoxyuridine, and the acceptor base is 5-trifluoromethyluracil.

In one embodiment, in the transglycosylation method of the presentinvention, the sugar-donating nucleoside is selected from uridine and2′-deoxyuridine, and the acceptor base is trans-zeatin.

In one embodiment, in the transglycosylation method of the presentinvention, the sugar-donating nucleoside is selected from uridine and2′-deoxyuridine, and the acceptor base is 2-chloro-6-methylaminopurine.

In one embodiment, in the transglycosylation method of the presentinvention, the sugar-donating nucleoside is selected from uridine and2′-deoxyuridine, and the acceptor base is 6-dimethylaminopurine.

In one embodiment, in the transglycosylation method of the presentinvention, the sugar-donating nucleoside is selected from uridine and2′-deoxyuridine, and the acceptor base is 6-mercaptopurine.

The term “thermostable nucleoside phosphorylase” refers to enzymes thatare stable to heat, are heat resistant, and retain sufficient nucleosidephosphorylase activity to effect another reactions and does not becomeirreversibly denatured (inactivated) when subjected to elevatedtemperatures for the time necessary to effect the transglycosylationreactions.

The purpose of the Examples given below is to illustrate the presentinvention without constituting a limitation of the field of applicationthereof.

EXAMPLES

As stated above, the transglycosylation reactions according to theinvention were carried out using UPase and PNPase enzymes obtained fromUPase- or PNPase-producing cells. Such cells are preferably cells ofgenetically modified Escherichia coli, capable of expressingconsiderable quantities of UPase or PNPase. The manner of obtaining suchcells, the enzymes and their characteristics, as well as the productionof some nucleoside analogues are given in the accompanying examples,which are set out below.

Example 1 Construction of the Escherichia coli deoD

The Purine Nucleoside Phosphorylase (PNPase) of Sulfolobus solfataricussequence was found in GenBank, with the accession number AE006766. Thegene was amplified by PCR, using 2 units of Platinum Pfx enzyme(Invitrogen), 1 mM MgSO₄ and 1× enzyme amplification buffer, 200 μMdNTPs and 0.3 μM of each primer, with the oligonucleotides5′-caccgtgccatttttagaaaatggttcc-3′ (Sulfolobus solfataricus deoDforward; SEQ ID NO. 1) and 5′-aatcagttttaagaatcttaaggtaat-3′ (Sulfolobussolfataricus deoD reverse; SEQ ID NO. 2) from the Sulfolobussolfataricus P2 [15]. The PCR reaction was performed with an initialdenaturation step at 94° C. for 30 min, followed by 36 temperaturecycles of a denaturation step at 94° C. for 1 min, and anannealing/extension step at 60° C. for 1.5 min and 68° C. for 1 min.After the 36 cycles, the sample was subjected to 68° C. for 10 min andfinally at 4° C. PCR product was analyzed by agarose gelelectrophoresis, and the DNA band was purified from the gel (S.N.A.P.™UV-Free Gel Purification Kit, Invitrogen). The amplified fragment wascloned into the polylinker region of the pUC18 vector that carries theampicillin resistance gene [17]. The cloned region was completelysequenced and it was found to be completely identical to the data banksequence.

Example 2 Construction of the Escherichia coli Udp

The Pyrimidine Nucleoside Phosphorylase (UPase) of Aeropyrum pernixsequence was found in GenBank with the accession number NC_(—)000854.2.The gene was amplified by PCR, using 2 units of Platinum Pfx enzyme(Invitrogen), 1 mM MgSO₄ and 1× enzyme amplification buffer, 200 μMdNTPs and 0.3 μM of each primer, with the oligonucleotides5′-caccgtggcccgctacgttctcctc-3′ (Aeropyrum pernix udp forward; SEQ IDNO. 3) and 5′-gaattcctatgtgcgtctgcacgccagg-3′ (Aeropyrum pernix reverse;SEQ ID NO. 4) from the Aeropyrum pernix K1 [16]. The PCR reaction wasperformed with an initial denaturation step at 94° C. for 30 min,followed by 36 temperature cycles of a denaturation step at 94° C. for 1min, and an annealing/extension step at 60° C. for 1.5 min and 68° C.for 1 min. After the 36 cycles, the sample was subjected to 68° C. for10 min and finally at 4° C. PCR product was analyzed by agarose gelelectrophoresis, and the DNA band was purified from the gel (S.N.A.P.™UV-Free Gel Purification Kit, Invitrogen). The amplified fragment wascloned into the polylinker region of the pUC 18 vector that carries theampicillin resistance gene [17]. The cloned region was completelysequenced and it was found to be completely identical to the data hanksequence.

Example 3 Cloning into pET102/D-TOPO® Vector and Cells Transformation

The DNA fragments containing the Aeropyrum pernix udp gene sequence andSulfolobus sulfactaricus deoD gene sequence were cloned intopET102/D-TOPO® vector (pET102 Directional TOPO® Expression Kit,Invitrogen) that comprises a pBR322ori for plasmid replication,ampicillin resistance gene, T7 promoter that permits binding of T7 RNApolymerase, lac operator that permits inhibition of expression whenisopropylthiogalactopyranoside (IPTG) is not present, a ribosome bindingsite for translation of RNA, a His-patch-thioredoxin for increasing thesolubility of the fusion protein, and a polyhistidine tag (6×His) fordetection and purification of the fusion protein (FIG. 3). The vectorwas introduced by thermal shock into Escherichia coli BL21 (pET102Directional TOPO® Expression Kit, Invitrogen), a commercial strain thatis used for regulated expression of heterologous genes. It comprises thegene encoding the T7 RNA polymerase, which makes the strain compatiblewith the use of pET vectors comprising T7 promoter for theoverexpression of recombinant proteins with IPTG.

The resulting recombinant vectors were analyzed by DNA restriction with10 units of the enzyme HindIII. The positive clone was sequenced usingthe sequencing primers TrxFus forward 5′TTCCTCGACGCTAACCTG3′ (SEQ ID NO.5) and T7 reverse 5′TAGTTATTGCTCAGGGGTGG3′ (SEQ ID NO. 6).

Example 4 Fermentation of the Recombinant Strains

The recombinant strains to which the present invention relates werecultivated separately in batch mode, at pH=7, in trypticase soy solidmedium supplemented with ampicillin. One colony of the culture waspassed into nutrient broth n° 2 (Oxoid) containing Lab-lemco powder 10g/l, peptone 10 g/l and NaCl 5 g/l, supplemented with 200 mg/lampicillin. It was incubated at 37° C. with vigorous shaking (200 rpm).Escherichia coli strains harbouring the expression plasmid were grown toOptical Density at 600 nm of 0.6, then IPTG (up to 100 mg/l) was addedand the culturing was continued for an additional 8 h. When fermentationwas complete, the culture medium was centrifuged, the cell pellet waswashed in 30 mM-pH 7 phosphate buffer. The biomass obtained was storedat −20° C. until it was brought into use.

Example 5 Partial UPase and PNPase Purification (Cell LysatePreparation)

For a partial purification of the protein, cell paste, separated bycentrifugation or by microfiltration from a culture of the recombinantstrain that expresses the enzyme UPase or PNPase, was disrupted by theaddition of 160 mg lysozyme and one hour of incubation at 0° C. followedby three ultra rapid freezing-thawing cycles (−80° C./37° C.) and afinal step for reducing viscosity adding 1,000 units ofdeoxyribonuclease 1.

Example 6 Determination of the Enzymatic Activity of the UPase Enzyme

One hundred microliters of centrifuged cell lysate, comprising asuspension of UPase expressing cells, diluted 1:100 (volume/volume) inpotassium phosphate buffer at pH 7.0, was added to 800 microliters of 75mM uridine solution in 100 mM phosphate buffer at pH 7.0, preincubatedat 30° C. After 5 minutes, the phosphorolysis reaction was stopped byaddition of 1 ml of 2N HCl. An aliquot of the reaction mixture wasanalyzed using a high performance liquid chromatograph (HPLC), equippedwith a Kromasil 100-5C18 (Akzo Nobel) column, with a size of 250×4.6 mm.The elution was carried out using a 4% methanol-water solution. Theenzymatic activity of the cell lysate was expressed as units per ml(μmoles of uracil×min⁻¹×ml⁻¹) and was calculated relative to a standarduracil solution eluted in the same conditions. Approximately 590 unitsper ml of centrifuged cell lysate were recovered.

Example 7 Determination of the Enzymatic Activity of the PNPase Enzyme

One hundred microliters of centrifuged cell lysate, comprising asuspension of PNPase expressing cells, diluted 1:100 (volume/volume) inpotassium phosphate buffer at pH 7.0, was added to 800 microliters of 60mM inosine solution in 100 mM phosphate buffer at pH 7.0, preincubatedat 30° C. Exactly 10 minutes later, the phosphorolysis reaction wasstopped by addition of 1 ml of 2N HCl. An aliquot of the reactionmixture was analyzed using a high performance liquid chromatograph(HPLC), equipped with a Kromasil 100-5C18 (Akzo Nobel) column, with asize of 250×4.6 mm. The elution was carried out using a 4%methanol-water solution. The enzymatic activity of the cell lysate wasexpressed as units per ml (μmoles of hypoxanthine×min⁻¹×ml⁻¹) and wascalculated relative to a standard hypoxanthine solution eluted in thesame conditions. Approximately 310 units per ml of centrifuged celllysate were recovered.

Example 8 Determination of Transglycosylation Catalytic Activity

For the determination of transglycosylation catalytic activity themixture of cell lysates containing both UPase and PNPase were preparedby mixing the lysates so as to have UPase:PNPase enzymatic-activityratio of about 1:1, determined as in Examples 6 and 7.Transglycosylation reaction was carried out at analytical scale in thefollowing conditions: 250 μl of cell lysates (equivalent to 14 units ofeach UPase and PNPase enzymatic activities) was added to 10 ml of asolution having the following composition: 4 mM1-β-D-ribofuranosyluracil (uridine nucleoside), 4 mM adenine base, 30 mMpotassium phosphate buffer pH7, thermostatically controlled at 60° C.After 1.5 hours at 60° C., the reaction was stopped by diluting themixture 1:5 and cooling in ice. The percentage of bioconversion ofadenine base to 9-β-D-ribofuranosyladenine (adenosine nucleoside) wasdetermined by analyzing an aliquot of the reaction mixture by highperformance liquid chromatography (HPLC) with the use of Kromasil100-5C18 (Akzo Nobel) column of a size of 250×4.6 mm and eluted with a4% methanol-water solution. The transglycosylation catalytic activitywas expressed as units·ml⁻¹ (μmoles of Ara-A formed in 1.5 hours·ml⁻¹ ofmixture of cell lysates) or in units·g⁻¹ of moist resin (μmoles ofD-ribofuranosyladenine formed in 1.5 hours·ml⁻¹ of cell lysate) and wascalculated relative to a standard D-ribofuranosyladenine solution elutedby HPLC in the same conditions. Under these conditions, about 55 percentof adenosine nucleoside was formed (approximately 9 units per ml of celllysate).

Example 9 Effect of Temperature and pH on the Nucleoside PhosphorylasesActivities

The influence of pH and temperature on the performance of the nucleosidephosphorylases was investigated by using a “design of experimentsmethod”. Different temperatures were combined with different pH valuesto ascertain the conditions that resulted in maximum enzymaticactivities. An experimental domain was defined between 80° C.-100° C.and pH 5.5-8.5. As shown in FIG. 4, five temperatures were combined withthree pH values according to the “Doehlert Matrix”, resulting in sevencombinations of temperature and pH. The substrates were incubated in theselected reaction solution and temperature without enzyme, and then theenzyme was added and incubated at selected conditions. The samples wereprocessed as described above for the determination of the enzymaticactivity of the PNPase or UPase enzymes. Activity values were expressedas a percentage of the corresponding maximal values found (100%). Theresults are shown in FIGS. 5 and 6, respectively.

Example 10 Thermostability

Thermostability of the enzymes was assayed at 80° C. Aliquots of theenzyme were prepared, between 100 and 200 microliters of a suspension ofUPase or/and PNPase expressing cells (cell lysate), diluted 1:100 or1:1000 volume/volume in potassium phosphate buffer at pH 7.0-7.2. Thebiocatalysts were incubated at 80° C. for different times. Afterheating, the solutions were immediately kept on ice and the measuring ofthe nucleoside phosphorylases activities was performed as described inexamples 6 and 7. No loss of nucleoside phosphorylases activities wasobserved after 10 h at 80° C.

Solvent Effects on Nucleoside Phosphorylase Activity

Organic co-solvents are commonly used in reactions and isolated enzymesmust be able to survive under conditions of relatively highconcentrations of co-solvent. Experiments were run in the presence oforganic solvents such as methanol (protic polar solvent) anddimethylsulfoxide (aprotic polar solvent). The nucleoside phosphorylaseenzymes of the present invention were resistant to organic solvents,exhibiting activity in a buffer solution containing 5 and 10% by volumeof the organic solvent.

TABLE 1 Stability of PNPase enzyme against some organic solvents.Relative activity (%)^(a) Organic solvents DMSO MeOH  5% (v/v) 94 95 10%(v/v) 115 86 ^(a)The relative activity of cell lysate comprising PNPaseenzyme was determined using the standard assay described in example 7 attwo concentrations of organic solvents and its relative activity wascompared with the activity of the cell lysate comprising PNPase enzymebut no organic solvent. DMSO: dimethylsulfoxide; MeOH: methanol.

TABLE 2 Stability of UPase enzyme against some organic solvents.Relative activity (%)^(a) Organic solvents DMSO MeOH  5% (v/v) 91 15110% (v/v) 70 206 ^(b)The relative activity of cell lysate comprisingUPase enzyme was measured using the standard assay described in example6 at two concentrations of organic solvents and its relative activitywas compared with the activity of the cell lysate comprising UPaseenzyme but no organic solvent. DMSO: dimethylsulfoxide; MeOH: methanol

Example 11 General Bioconversion Procedures

One-pot transglycosylation reactions were performed at selected reactiontemperatures and pHs, in reaction vessels with gentle stirring.Catalytic transglycosylation activity of cell lysate mixture used wasabout 12 units per ml (as determined in Example 8). Nucleosides wereprepared using a mixture of the cell lysate prepared to obtain aPNPase:UPase activity ratio of 1:1.

Substrates were:

1) natural 2′-deoxyribonucleosides or ribonucleosides, which act as2′-deoxyribose-1-phosphate or ribose-1-phosphate donors;2) a secondary natural or synthetic purine or pyrimidine base, which islinked to the position 1 of the sugar.

At the end of the reactions, the reaction mixture was centrifuged andthen filtered using an Amicon Ultrafiltration device (YM-3 membrane) andthe products were separated, according to the protocol provided by themanufacturer. The biocatalysts can be recycled for consecutive reactions(typically between 3-5 times) by addition of newly prepared reactionmixtures. The filtered solutions were monitorized by high performanceliquid chromatography (HPLC) in the same conditions described above.Bioconversion yields were calculated based on the initial concentrationof base analogue.

Example 12 Preparation of 2,6-Diaminopurine Nucleosides

The preparation of purine nucleosides is more difficult because of thelow solubility of these bases in aqueous medium. In the presentinvention, the inventors used the enzyme competitive properties to solvethis problem. The strategy used by the inventors encompassed the use oforganic co-solvents and/or high temperatures to increase the solubilityof the substrates.

12.1 Preparation of 2,6-Diaminopurine Nucleosides Using Co-Solvents

Assays were performed in 5 and 10% of aprotic dipolar co-solvents, at60° C., twelve units/ml cell lysate were added to 150 ml of a solutionkept thermostatically at 60° C., and having the following composition ofsubstrate solutions:

1. 15 mM Uridine/2′-Deoxyuridine, 2. 5 mM 2,6-Diaminopurine, and

3. 30 mM potassium phosphate buffer, pH 7.

After 5 hours, the reaction mixture was filtered by centrifugation at2000×g for 30 min, at 4° C., through an Amicon ultra-4 CentrifugalFilter Devices (Millipore, Bedford, Mass.) with a 3000-Da cutoff, andthe filtrate was recovered. In table 3, the production yields of2,6-Diaminopurine nucleosides prepared in the presence of co-solvents isshown. The resulting 2,6-Diaminopurine nucleosides (2,6-Diaminopurineriboside or 2,6-Diaminopurine deoxyriboside) were analyzed by HPLC. Inthese reactions, the products were obtained in high yields (over 80%).

TABLE 3 Production yields of 2,6-Diaminopurine nucleosides usingdifferent % co- solvent at 60° C. Yield Co-solvent Substrate Analog baseProduct (%) 10% DMSO Uridine 2,6- 2,6- 97 Diaminopurine Diaminopurineriboside 10% DMSO 2′- 2,6- 2,6- 96 Deoxyuridine DiaminopurineDiaminopurine deoxyriboside  5% THF Uridine 2,6- 2,6- 84 DiaminopurineDiaminopurine riboside  5% THF 2′- 2,6- 2,6- 90 DeoxyuridineDiaminopurine Diaminopurine deoxyriboside THF: Tetrahydrofuran; DMSO:dimethylsulfoxide

12.2. Preparation of 2,6-Diaminopurine Nucleosides Without the Use ofCo-Solvents

The preparation of 2,6-Diaminopurine nucleosides without co-solventscould only be carried out at temperatures above 80° C. due to limitedwater solubility of this purinic base. Assays were performed atdifferent temperatures (80, 90 and 100° C.). Twelve units/ml cell lysatewere added to 150 ml of a solution kept thermostatically at the selectedtemperature, and having the following composition of substratesolutions:

1. 15 mM Uridine/2′-Deoxyuridine, 2. 5 mM 2,6-Diaminopurine, and

3. 30 mM potassium phosphate buffer, pH 7.

After 5 hours, the reaction mixture was filtered by centrifugation at2000×g for 30 min, at 4° C., through an Amicon ultra-4 CentrifugalFilter Devices (Millipore, Bedford, Mass.) with a 3000-Da cutoff, andthe filtrate was recovered. In table 3, the production yields of2,6-Diaminopurine nucleosides prepared in the presence of co-solvents isshown. The resulting 2,6-Diaminopurine nucleosides (2,6-Diaminopurineriboside or 2,6-Diaminopurine deoxyriboside) were analyzed by HPLC. Intable 4, the production yields of 2,6-Diaminopurine nucleosides preparedwithout organic solvents are shown. The bioconversion yield of thereaction was around 80% in all temperatures assayed.

TABLE 4 Production yields of 2,6-Diaminopurine nucleosides at differenttemperatures without use organic co-solvents. Temperature (° C.) 80 90100 Biotransformation (%) 79.3 79.6 81.1

Example 13 Preparation of Other Nucleosides

Transglycosylation reactions were carried out using various acceptorbases. The reactions were carried out at 80° C. for various periods oftime with 10% of DMSO as co-solvent for purinic base and withoutco-solvent use for pyrimidinic aceptor bases. The percentage ofbioconversion was calculated relative to the initial concentration ofacceptor base and was determined by HPLC analysis of the reactionmixture.

13.1 Preparation of Trifluoromethyluracil Nucleosides

One ml catalyst (cell lysate) with transglycosylation activity of about12 units/ml cell lysate were added to 150 ml of a solution keptthermostatically between 80° C., and having the following composition:

-   -   7.5 mM uridine/2′-Deoxyuridine,    -   2.5 mM Trifluoromethyluracil, and    -   20 mM potassium phosphate buffer, pH 7.

After 3 hour at 80° C., the reaction mixture was filtered bycentrifugation at 2000×g for 30 min, at 4° C., through an Amicon ultra-4Centrifugal Filter Devices (Millipore, Bedford, Mass.) with a 3000-Dacut-off, and the filtrate was recovered. The bioconversion yield of thereaction was higher than 60%. The resulting trifluoromethyluracilnucleosides (trifluoromethyluridine or 7′-deoxytrifluoromethyluridine)were analyzed by HPLC.

13.2 Preparation of 5-Fluoruracil Nucleosides

5-Fluoruracil nucleosides were prepared as previously described above.One ml catalyst (cell lysate) with transglycosylation activity of about12 units/ml cell lysate was added to 150 ml of a solution keptthermostatically at 80° C. and having the following composition:

-   -   7.5 mM uridine/2′-Deoxyuridine,    -   2.5 mM 5-fluorouracil, and    -   30 mM potassium phosphate buffer, pH 7.

After 3 hour at 80° C., the reaction mixture was filtered bycentrifugation at 2000×g for 30 min, at 4° C., through an Amicon ultra-4Centrifugal Filter Devices (Millipore, Bedford, Mass.) with a 3000-Dacut-off, and the filtrate was recovered. The bioconversion yield of thereaction was higher than 50%. The resulting 5-fluoruracil nucleosides(5-fluorouridine and 5-fluoro-2′-deoxyuridine) were analyzed by HPLC.

13.3 Preparation of trans-Zeatin Nucleosides

trans-Zeatin nucleosides were prepared as described above. One mlcatalyst (cell lysate) with transglycosylation activity of about 12units/ml cell lysate was added to 150 ml of a solution keptthermostatically at 80° C. and having the following composition:

-   -   7.5 mM Uridine/2′-Deoxyuridine,    -   2.5 mM trans-Zeatin, and    -   30 mM potassium phosphate buffer, pH 7.

The assays were performed in 10% (v/v) DMSO as co-solvent using theconditions described above. After 5 hours at 80° C., the reactionmixture was filtered by centrifugation at 2000×g for 30 min, at 4° C.,through an Amicon ultra-4 Centrifugal Filter Devices (Millipore,Bedford, Mass.) with a 3000-Da cut-off, and the filtrate was recovered.The bioconversion yield of the reaction was higher than 80%. Theresulting trans-zeatin nucleosides (trans-zeatin riboside andtrans-zeatin deoxyriboside) were analyzed by HPLC.

13.4 Preparation Process for 2-Chloro-6-methylaminopurine Nucleosides

2-Chloro-6-methylaminopurine nucleosides were prepared as describedbefore. One ml catalyst (cell lysate) with transglycosylation activityof about 12 units/ml cell lysate were added to 150 ml of a solution keptthermostatically at 80° C. and having the following composition:

-   -   15 mM Uridine/2′-Deoxyuridine,    -   5 mM 2-Chloro-6-Methylaminopurine, and    -   30 mM potassium phosphate buffer, pH 7.

The assays were performed in 10% (v/v) DMSO as co-solvent using theconditions described above. After 5 hours at 80° C., the reactionmixture was filtered by centrifugation at 2000×g for 30 min, at 4° C.,through an Amicon ultra-4 Centrifugal Filter Devices (Millipore,Bedford, Mass.) with a 3000-Da cut-off, and the filtrate was recovered.The bioconversion yield of the reaction was higher than 80%. Theresulting 2-chloro-6-methylaminopurine nucleosides(2-chloro-6-methylaminopurine riboside and 2-chloro-6-methylaminopurinedeoxyriboside) were analyzed by HPLC.

13.5 Preparation of 6-Dimethylaminopurine Nucleosides

6-Dimethylaminopurine nucleosides were prepared as described above. Oneml catalyst (cell lysate) with transglycosylation activity of about 12units/ml cell lysate was added to 150 ml of a solution keptthermostatically at 80° C. and having the following composition:

-   -   15 mM Uridine/2′-Deoxyuridine,    -   5 mM 6-Dimethylaminopurine, and    -   30 mM potassium phosphate buffer, pH 7.

The assays were performed in 10% (v/v) DMSO as co-solvent using theconditions described above. After 5 hours at 80° C., the reactionmixture was filtered by centrifugation at 2000×g for 30 min, at 4° C.,through an Amicon ultra-4 Centrifugal Filter Devices (Millipore,Bedford, Mass.) with a 3000-Da cut-off, and the filtrate was recovered.The bioconversion yield of the reaction was higher than 80%. Theresulting 6-dimethylaminopurine nucleosides (6-dimethylaminopurineriboside and 6-dimethylaminopurine deoxyriboside) were analyzed by HPLC.

13.6 Preparation of 6-Mercaptopurine Nucleosides

6-Mercaptopurine nucleosides were prepared as described above. One mlcatalyst (cell lysate) with transglycosylation activity of about 12units/ml cell lysate was added to 150 ml of a solution keptthermostatically at 80° C. and having the following composition:

-   -   15 mM Uridine/2′-Deoxyuridine,    -   5 mM 6-Mercaptopurine, and    -   30 mM potassium phosphate buffer, pH 7.

The assays were performed in 10% (v/v) of DMSO as co-solvent using theconditions described above. After 5 hours at 80° C., the reactionmixture was filtered by centrifugation at 2000×g for 30 min, at 4° C.,through an Amicon ultra-4 Centrifugal Filter Devices (Millipore,Bedford, Mass.) with a 3000-Da cut-off, and the filtrate was recovered.The bioconversion yield of the reaction was higher than 50%. Theresulting 6-mercaptopurine nucleosides (6-mercaptopurine riboside and6-mercaptopurine deoxyriboside) were analyzed by HPLC.

All publications, patents, and patent applications cited in thisdocument are incorporated by reference herein in their entirety, asthough individually incorporated by reference.

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1-13. (canceled)
 14. A recombinant expression vector comprising: thesequence encoding a uridine phosphorylase (UPase, EC. 2.4.2.3); saidsequence operably linked to one or more control sequences that directthe production of said phosphorylase in a suitable expression host; saidsequence originating from the Archaea Thermoprotei class, wherein theUPase is from Aeropyrum pernix (SEQ ID No. 8).
 15. A host cellcomprising the recombinant expression vector according to claim
 14. 16.The host cell according to claim 15, wherein said host cell isEscherichia coli.
 17. The host cell according to claim 15, wherein thehost cell is in the form of a lysate.